Cores of power-circuit reactor can be broadly divided into three types. In the region of not more than several tens of kHz, silicon steel sheets, amorphous soft magnetic sheet strips, nanocrystalline soft magnetic sheet strips and the like are mainly used as core materials. These core materials contain iron as a main component and have the advantage that the saturation magnetic flux density Bs and magnetic permeability μ are great. However, silicon steel sheets have the disadvantage that high-frequency core losses are great, and amorphous soft magnetic sheet strips and nanocrystalline soft magnetic sheet strips have the disadvantage that the core shape is restricted by the wound core shape, the laminated core shape and the like, and cannot be easily shaped into such various shapes as those made of ferrite, which will be described later.
In the region of not less than several tens of kHz, ferrite cores represented by Mn—Zn ferrite and Ni—Zn ferrite are widely used. The ferrite cores have small high-frequency core losses and molding is relatively easy and, therefore, the ferrite cores have the advantage that various shapes of cores can be mass produced. However, because the saturation magnetic flux density Bs of the ferrite cores is as small as about ¼ to ½ of those of the above-described silicon steel sheets, amorphous soft magnetic sheet strips and nanocrystalline soft magnetic sheet strips, the sectional core area is large in order to avoid magnetic saturation in large-current reactors.
There are powder magnetic cores for use in the range from several kHz to several hundreds of kHz. Powder magnetic cores are obtained by working and molding after the surface of magnetic powders is subjected to an insulating process, and the occurrence of eddy current losses is suppressed by the insulating process.
Hybrid electric vehicles that have recently begun to rapidly come into widespread use have a large-output electric motor, and a reactor capable of withstanding a high voltage and a large current is used in a power circuit that drives this electric motor. The reactor is also used in a power conditioner and the like. Requirements for small designs, low-noise designs and low-loss designs are strong to the reactor, and core materials used in the reactor are required to provide high saturation magnetic flux densities Bs and magnetic permeability μr, in an appropriate range as magnetic properties. A description will be given below of the magnetic permeability μr, in an appropriate range mentioned here.
The relationship B=μ0μrH exists between a magnetic field H and magnetic flux density B. In this expression, μ0 denotes the magnetic permeability in a vacuum and the magnetic field H is proportional to a current flowing in a reactor. For this reason, in a core material of high magnetic permeability, the saturation magnetic flux density Bs is reached even in the case of a small reactor current, thereby causing core saturation. Therefore, designing has hitherto been carried out in such a manner that a magnetic material having a high saturation magnetic flux density Bs is used as the reactor core material, the effective magnetic permeability μre is reduced by providing gaps in this core material, and a necessary inductance is obtained by adjustment with respect to the number of windings. For example, the effective magnetic permeability μre that is practicable in reactors for hybrid electric vehicles is in the range of approximately 10 to 50, and the effective magnetic permeability μre that is practicable in power conditioner reactors is in the range of approximately 30 to 100. Therefore, it is desirable to use the above-described powder magnetic cores.
In a large-current reactor core, magnetic materials having a high saturation magnetic flux density Bs and low losses are used. In general, magnetic materials having a high saturation magnetic flux density Bs and low losses also have a high magnetic permeability and, therefore, gaps are provided when these magnetic materials are used in the reactor core. Because the magnetic permeability of members that constitute the gaps is approximately 1, in the gaps is generated a fringing magnetic flux in which the magnetic flux leaks out to the outside of the magnetic path. For this reason, an eddy current is generated on the coil surface in the vicinity of the gaps, thereby posing the problem that losses increase.
For example, Japanese Patent Laid-Open No. 2005-50918 (Patent Document 1) discloses as an example an annular reactor core that is a powder magnetic core. In this reactor core, in order to suppress an increase in loss due to a fringing magnetic flux, a multiple-gap structure in which the gap length per place is reduced is used, and a reactor core having gaps in a total of six places is described. Also, a reactor core having gaps in a total of eight places is disclosed in Japanese Patent Laid-Open No. 2005-19764 (Patent Document 2) etc. Furthermore, Japanese Patent Laid-Open No. 2003-45724 (Patent Document 3) discloses as an example an annular reactor core in which core legs are integrally made. This reactor core is based on the premise that for the core legs, sheet materials are blanked and laminated, and that a study is carried out on the bonding strength and the number of laminations, which are used when the blanked materials are laminated.